The insulation of pipes using rigid polyurethane foams by the technique of rotational casting is known, for example, from German Offenlegungschrift 4,118,362. In practice, however, the formulations used are greatly in need of improvement. Extreme demands are made on the reaction mixtures in connection with the rotational casting process. On one hand, the reacting foaming material must still rise sufficiently against the rotating tube and flow as a single strand. On the other hand, it must not flow off the tube or from the applicator nozzle at the beginning due to inadequate cross-linking. This is further aggravated by the fact that the result is to be a dimensionally stable foamed material having an almost uncorrugated surface, whereon a thermoplastic top coating is frequently applied. In the case of undulating surfaces, the top coating flows into the troughs of the waves, whereby a thinner top coating forms on the crest of the waves. Not only is an optically uneven top coating obtained, but the mechanical endurance and the barrier behavior against diffusion also suffer.
The bonding of the foamed material to the pipe to be insulated has proved to be another important property in technical use. This is especially critical, because an uncompressed foamed material is involved. Furthermore, the foaming agent components employed should comply with recent ecological findings and be CFC-free.
An operation which runs without problems for a longer period was still not provided by the process of rotational casting described in German Offenlegungschrift 4,118,362. The reaction mixture leading to the rigid polyurethane foam, for example, suddenly no longer exhibits flowability after a short machine operation of approximately 7 minutes, which results in clogging of the discharge nozzle. A continuous processing is consequently impossible. However, the continuous and constant rotational coating of pipes of any length and the subsequent coating thereof with a thermoplastic covering is nevertheless a requirement for technical use.
Surprisingly, the use according to the invention of two different cross-linking agents as reagents in the production of rigid polyurethane foams for the rotational casting process has proved successful. The foams also possess good tensile and shear properties.